1. Field of the Invention
The present invention relates to an epitaxial material used for GaN based LED and a method for manufacturing the same, particularly to an epitaxial material used for GaN based LED with low polarization effect by using quantum well regulating and controlling technology to reduce the polarization effect and a method for manufacturing the same.
2. Description of the Related Art
Wide-bandgap materials with GaN as a representative are third generation semiconductors successive of Si and GaAs semiconductors, which is developed rapidly in the nineties of the last century due to the breakthrough of the epitaxial growing technology. Within less than ten years, GaN becomes a very hot subject for study worldwidely, while the LED possesses a major part among the GaN market. Presently, it had been already industrialized to utilize C-plane and A-plane sapphire substrate for fabricating GaN based LED. Normally we use epitaxial technology to grow GaN based LED on C-plane sapphire substrate to obtain C-plane GaN. Because III-V Nitride material has a spatial structure without space inversion symmetry, and the electronegativity of atom of V group elements is far different from that of N, there is a strong polarization along <0001> of GaN, thus the polarization effect will generate a built-in electric field with rather high intensity, and spatially separate positive and negative charge carriers, which results in red shift of emission wavelength, and even worse reduction in wave function overlap of electron and hole, so as to greatly decrease the light emission efficiency of the material.
Recently, the highest inner quantum efficiency of GaN based blue LED in the world market reaches 40-60% only, and commonly only 20%. While the LED of GaAs system without polarization effect can reach 100% highest inner quantum efficiency, with the common products reaching about 70% inner quantum efficiency. That is why the study of growing GaN with non-polarized plane and cancelling the spatial separation of electron and hole becomes one very important approach for increasing light emission efficiency of LED.
Because of the poor crystal quality of GaN, its dislocation density reaches 108-1010/cm−2, and its strong polarization effect severely restrains the light emission efficiency of LED. It is demonstrated in experiment that adding In in quantum well can effectively increase the light emission efficiency. The current main aspect believes that existence of In component results in a localization effect, which in turn increases the light emission efficiency of quantum well, However, due to the existence of the polarization effect, the effect of increasing the light emission efficiency caused by the localization effect is cancelled out to a great extent. The analysis shows that GaN based LED material with non-polarization effect will substantially increase the light emission efficiency of the quantum well, so as to essentially solve the problem of how to obtain GaN based LED with high efficiency.
Currently, there are two solutions for growing GaN based quantum well with non-polarization effect: the first one is to select substrate with non-polarized plane to grow the material without polarization effect; the second one is to grow AlInGaN quaternary alloy material, the compositions of which is suitably selected in order to counteract the polarization effect. The first solution has two ways, one is to grow (1-100) M-plane GaN on (001) substrate of γ-LiAlO2 by MBE technology, the main difficulty of which is the poor thermal stability of γ-LiAlO2 and high dopant concentration of the grown GaN material background, another one is to grow non-polarized A-plane GaN material on R-plane sapphire substrate, the main difficulty of which is that the A-plane GaN epitaxial material presents severe non-symmetry, the epitaxial material can hardly grow in two-dimensional layer status. A difficulty of the second solution is that it is hard to grow a material completely counteracting the polarization effect.
Therefore, it is really desirable to introduce an improved method for growing an epitaxial material for GaN based LED with low polarization effect.